1. Field of the Invention
The present invention relates to a manufacturing method of a semiconductor device and more particularly to a method of pattern formation using a phase-shift mask.
2. Description of the Related Art
For the purpose of achieving a higher integration in a semiconductor device, formation of a minuter and more densely spaced pattern in the step of photolithography has been being sought after. Accordingly, while the exposure method with reduced projection is, in general, currently used for the exposure in the step of photolithography, the phase-shift method is employed therein so as to raise the limit of resolution further and bring about formation of a still minuter and more densely spaced pattern.
The basic principle of the phase-shift method is briefly described below, taking the case of forming the line and space (L/S) patterns by the Levenson-type phase-shift method. In this case, there is utilized an alternating phase-shift mask in which lines are formed of a light-shielding material on a transparent substrate and phase shifters are placed every other opening sections (space sections). The light passing through an opening section of this mask travels through a lens and produces an image on a resist film lying on a wafer. If the distance between two neighboring opening sections becomes considerably short, when the normal mask without a phase shifter is used, the diffracted lights traveling from these two neighboring opening sections are equal in phase so that, through their mutual interference, the images of these two neighboring opening sections cannot separate from each other. In contrast with this, with a phase-shift mask, phase difference between two lights from each neighboring opening thereof is 180 degrees and so, the two lights interfere each other, diffracted lights are destroyed and, consequently, the images of these two neighboring opening sections separate from each other.
The phase-shift method of this sort is used for the formation of repetition patterns such as the L/S patterns, solitary patterns, random patterns and the likes and applied to the fabrication of various semiconductor devices including bit lines of a DRAM (Dynamic Random Access Memory) and gate patterns of a CMOS (Complementary Metal-Oxide-Semiconductor). For example, in Proc. SPIE (Proceedings of Society of Photo-optical Instrumentation Engineering), Vol. 3051, pp. 342-351 (1997), there is reported a study in which the phase-shift method is applied to obtain 0.16 xcexcm CMOS gate patterns.
The general method to form minute gate patterns by the phase-shift method is described below.
In this method, two exposures are made using two different masks shown in FIGS. 2(a) and (b) separately for respective exposures and, with a composite image obtained from the exposures through these two masks, gate patterns are formed on a wafer. FIG. 2(a) is a view showing the layout of a phase-shift mask (mask A) in which light-shielding sections for lines 22 and phase shifter sections 23 are formed on a transparent substrate 21. The patterns seen in the light-shielding sections for the lines 22 are the very patterns required to be formed. FIG. 2(b) is a view showing the layout of the other mask (mask B) in which a light-shielding section for protection 24 is formed in order to protect, against the second exposure, portions of a positive resist that are placed in the position of the gate patterns required to be formed, after the first exposure with the mask A is performed.
FIG. 3 presents patterns which are transcribed on the positive resist lying on a wafer, using the aforementioned mask A, mask B or both. FIG. 3(a) shows transcription patterns obtained by the first exposure with the mask A. Ring-shaped patterns are therein transcribed, which indicates patterns other than those formed by the light-shielding sections for the lines 22 are also transcribed. The sections of each ring-shaped patterns are formed by the edge of each phase shifter section. This results from a fact that the amplitude of the light is weakened at the edge sections around the phase shifter sections 23 and, thus, the light is shielded in substance. FIG. 3(b) shows the positioning relation between patterns transcribed by the first exposure with the mask A and patterns transcribed by the second exposure with the mask B. At the time of the second exposure, the mask B must be aligned in such a way that the portions of the resist placed in the position of the gate patterns required to be formed are well protected by the light-shielding section for protection 24. FIG. 3(c) showed resist patterns obtained, after the second exposure following the first exposure is made and then a subsequent development is carried out. Since irradiation is applied, at the second exposure, to the superfluous part of the patterns that are transcribed at the first exposure by shielding the light, the resist in that part is removed by the development, and thereby the prescribed resist patterns are formed.
Next, a method in which resist patterns are formed using two different masks described above and then the gate patterns are formed on a silicon substrate is described. FIG. 4 is two sets of schematic cross-sectional views illustrating the steps of this formation method. FIGS. 4 (a1)-(a4) is cross-sectional views taken along the line Axe2x80x94Axe2x80x2 of FIG. 3(b) and FIGS. 4 (b1)-(b4) are cross-sectional views taken along the line Bxe2x80x94Bxe2x80x2 of FIG. 3(b).
First, upon a silicon substrate 101, a gate oxide film 102 is formed and thereon a polysilicon film 103 is formed. Further, over that, a positive photoresist film 105 is formed. A first exposure is then applied to this silicon substrate through the mask A (106) (FIGS. 4 (a1), (b1)). Regions of the resist corresponding to the edge sections of the phase shifter sections 23 and the light-shielding sections for the lines 22 in the mask A are not irradiated so that ring-shaped patterns of the unexposed regions shown in FIG. 3(a) are transcribed.
Next, the mask B (107) is aligned so as to make its transcription patterns take the positioning relation shown in FIG. 3(b) and, then, through this mask B, a second exposure is made (FIGS. 4 (a2), (b2)). By this second exposure, the part of patterns of the unexposed regions caused by the edge sections of the phase shifter sections 23 in the mask A is also irradiated.
Subsequently, by performing a development, resist patterns 109 taking the shape shown in FIG. 3(c) are formed (FIGS. 4 (a3), (b3)). Using these resist patterns as a mask, the polysilicon film 103 is then etched and thereafter the resist 109 which becomes redundant is removed, and thereby gate patterns 110 corresponding to the resist patterns 109 are formed (FIGS. 4 (a4), (b4)).
The conventional method described above, however, has the following problem. Namely, as the spacing of the patterns narrows, the thinning of the patterns becomes conspicuous. If the final dimensions of the patterns very much shift from the designed values thereof because of that, various problems such as a decrease in the yield, a lowering of the reliability, the deterioration of element characteristics and the like are brought about.
The reason why such a thinning of patterns takes place is described, taking the case of the manufacturing method described above. In FIG. 3(b), if the spacing of the patterns or the distance between the lines (W1) narrows, the margin (W2) between the mask A and the mask B becomes small. When the margin (W2) becomes considerably small like this, the unexposed resist sections 108 that should be protected become liable to be affected by stray light. Consequently, the width (W3) of the resist patterns formed after performing the development is reduced, or in other words, the thinning of the patterns takes place.
An object of the present invention is to provide a manufacturing method of a semiconductor device, wherein, in the step of forming a pattern, even for a minute pattern with a narrow spacing, no thinning of the pattern takes place and the pattern practically having the designed dimensions can be formed.
The present invention relates to a method of manufacturing a semiconductor device; which comprises the steps of:
forming a film of a hard mask material, on a pattern-forming film which is to be used to form a prescribed pattern, and then forming a photoresist film over said film of the hard mask material;
carrying out a first exposure using a first mask with a phase shifter and subsequently making a development;
etching said film of the hard mask material using the formed resist pattern as a mask;
forming a photoresist film so as to cover the formed hard mask pattern;
carrying out a second exposure using a second mask with a pattern which enables a portion of the photoresist covering only a required part of said hard mask pattern to remain after the exposure and the development, and subsequently making a development;
removing, by means of etching, an unrequired part of the hard mask which is not covered with any portion of said photoresist; and
etching said pattern-forming film using the remaining hard mask pattern as a mask.
Further, the present invention relates to a method of manufacturing a semiconductor device; which comprises the steps of:
forming a film of a hard mask material, on a pattern-forming film which is to be used to form a prescribed pattern, and then forming a photoresist film over said film of the hard mask material;
carrying out a first exposure using a second mask with a pattern which makes a required part in a resist region corresponding to a transcription pattern of a first mask that is to be used in a later step unexposed and makes an unrequited part therein exposed, and subsequently making a development;
etching said film of the hard mask material using the formed resist pattern as a mask;
removing said resist pattern and thereafter forming a photoresist film so as to cover the formed hard mask pattern;
carrying out a second exposure using a first mask with a phase shifter and subsequently making a development;
removing, by means of etching, an unrequired part of the hard mask using the formed resist patterns as a mask; and
removing said resist pattern, and thereafter etching said pattern-forming film using the formed hard mask pattern as a mask.
With the present invention, a semiconductor device having high reliability and excellent element characteristics can be produced in a high yield.